Modern telecommunication systems require extremely stable and accurate timing devices, with the overall size, operating temperature, power consumption, weight, and ruggedness of the device being critical parameters. Quantum frequency apparatus, such as atomic frequency standards, are used in such applications.
In quantum frequency apparatus of the type considered here, a quantum medium is excited by two sources of electromagnetic energy. The "quantum medium" can be any gaseous, vaporous, liquid or solid material that exhibits natural resonances (referred to herein from time to time as "selected reference frequencies,") and that can be used for the purpose of optically detecting its natural resonances. The first source of electromagnetic energy can be light energy that is used to optically excite the medium, and the second source of electromagnetic energy can be a coherent source of electromagnetic radiation that is used to excite a selected reference frequency (i.e., a "natural resonance") of the medium. The optical excitation takes place in such a way that the light from the first source is both absorbed and transmitted by the quantum medium, with the portion of the light that is transmitted depending on the frequency difference between the selected reference frequency and the frequency of the electromagnetic radiation from the second source. This type of process is referred to from time to time herein as an optically-detected quantum resonance.
Atomic frequency standards, for example, use natural resonances within atoms to keep time since the natural atomic resonances (selected reference frequencies) are more stable and less sensitive to environmental effects, such as temperature, pressure, humidity, vibration, acceleration, etc., than are macroscopic oscillators like pendulums and quartz crystals. This type of natural atomic resonance, operating at a selected reference frequency, is used as a highly stable frequency reference to which the frequency of a variable frequency oscillator, such as a quartz oscillator, can be electronically locked so the high stability and relative insensitivity to environmental perturbations of such a natural atomic resonance are transferred to the quartz oscillator.
The atomic frequency standards usually comprise a voltage controlled oscillator (VCO), and a physics package and associated electronics that maintain an accurate and stable VCO standard frequency on a long-term basis. The physics package and associated electronics are used to slave the VCO to the frequency of the atomic resonance, thereby reducing frequency drift due to oscillator aging and the effects of the environment on the oscillator.
In a passive (Rb or Cs) gas-cell atomic frequency standard, the physics package includes a gas-exciting light source, a transparent glass cell containing an atomic gas and situated in a microwave cavity, and a photo detector. The microwave cavity is commonly used to couple injected electromagnetic energy at about the selected reference frequency to the atoms of the atomic gas, such as Rb or Cs, within the transparent cell. The microwave cavity is designed to have a microwave resonant frequency substantially equal to the selected reference frequency to optimize the effect of the injected electromagnetic field on the atomic gas in the cell. The injected microwave electromagnetic field is generated by frequency multiplication and synthesis from the VCO output.
In operation the atomic gas within the transparent glass cell is excited by the light from the gas-exciting light source. The microwave electromagnetic energy injected into the microwave cavity interacts with the excited atoms within the transparent cell and varies the intensity of the light transmitted through the gas cell in a manner dependent on the difference between the injected microwave frequency and the selected reference frequency. The light intensity transmitted through the gas cell is sensed by the photo detector and the variation in light intensity is detected and converted by the photo detector into a physics package electrical signal output. The physics package thus provides a frequency discriminating electrical output signal dependent upon the difference between the injected microwave frequency (which has been synthesized from the VCO frequency) and the stable selected reference frequency, which is used to lock the VCO frequency to this selected atomic resonant frequency.
Starting in the 1980s, there has been considerable interest in the use of semiconductor diode lasers in atomic frequency standards In gas-cell atomic frequency standards, the rf-discharge lamp is replaced by a semiconductor diode laser to reduce size, lower power consumption and costs.
However, these lasers tend to have relatively high FM noise. This laser FM noise is converted into AM noise by the atoms of the physics package and appears as such on its electrical signal output. This AM noise resulting from the conversion of laser light FM noise plus the laser AM noise have an adverse effect on the short-term frequency stability of the atomic standard, which can easily be an order of magnitude higher than for a conventional lamp-pumped standard, depending on the type of the laser.
As described in the paper "Recent Progress in Laser-Pumped Rubidium Gas-Cell Frequency Standards", G. Mileti, et al., tested the possibility of reducing this AM noise in a passive way. With a beam splitter, the laser light was separated into two beams. One beam, the signal beam, was passed through a first resonance cell with microwave interrogation to provide a first physics package signal. The other beam, the sample beam, was passed through a second atomic gas cell (identical to the first one, but without microwave interrogation) to provide a second physics package output signal. Noise reduction was sought by subtracting the first and second signals produced by these two beams so that the correlated AM noise was cancelled while the signal remained. Two ovens and two atomic gas cells are needed in this scheme. Also, the noise cancellation is dependent on the extent to which the light absorption environments in these two cells are identical.